Mass Effect 3 Walkthrough Part 65: Geth and Quarian Standoff

Although the Reaper is dead, you
still have not faced the most challenging part of the Priority: Rannoch
mission. The geth and the quarians are still facing off in the sky above
Rannoch. With the Reaper destroyed, the synthetic geth have been freed from its
manipulative influence. However, the quarians have been at war with the geth
for decades, and this one victory is not going to bring them together. So, you
will find that somewhat of a standoff has occurred in the space above Rannoch.

Legion will suggest that you
upload the Reaper code to the rest of the geth, everywhere. These upgrades
would give the geth truly free will, and would make them more powerful as well.
With these upgrades, the geth would be able to come to your aid against the
Reapers. Obviously, this is a decision that the quarians, Tali included (if she
is still alive) will not support. An upgraded geth could easily wipe out the
entire quarian race, or so they say. If you want the support of the quarians,
you will deny the geth their upgrades and let the quarians wipe the species out
here and now.

There is only one way to gain the
allegiance of both the geth and the quarians, and it is so highly specific and dependent
upon your actions in the last game that many players will not be able to
achieve it. First, you must have kept Tali alive at the end of Mass Effect 2. You
will also have to have saved Tali from being exiled during Mass Effect 2 by
completing her loyalty mission. Then, you have to have taken on Legion’s
loyalty mission in Mass Effect 2. At the end of this mission, you should have
killed the heretics instead of trying to gain their help. In order for this to
work, Legion has to have lived through Mass Effect 2 as well, or else killing
the heretics will have been pointless. Finally, you will have to have talked
things out between Tali and Legion in Mass Effect 2. There was a certain point,
towards the end of the game, where either Tali or Legion would lose your
loyalty unless you had a high enough Paragon or Renegade rating. If you managed
to keep both of their loyalties, you will be one step closer to brokering peace
between the races.

In addition to these basic
guidelines from Mass Effect 2, you have to have beaten the Geth Fighter
Squadrons mission in this game, covered in an earlier guide. You also should
have completed the mission Rannoch: Admiral Koris, again covered in an earlier
guide, and at the end of this mission you should have saved Koris instead of
his troops. If you meet all of these conditions, you can create a treaty
between the warring races and take both fleets to battle against the Reapers.
If not, it’s up to you which one to take.

One small factual correction: "At the end of this mission, you should have killed the heretics instead of trying to gain their help" Actually, I was able to achieve this without killing the heretics, I rewrote their code in ME2. Their total War Asset points is 810 after getting them on my side. I believe most of the other mentioned requirements are infact necessary, but also very important is your renegate or paragon score going into this mission.

I found also that the option was not available if I skipped the "disable geth server" mission, and was available if I completed it first.

It is not possible to achieve peace between the Geth and the Quarian without playing ME and ME2 because you're missing important decisions made in the previous games. It's not so much about the needed Paragon/Renegade points but about a "decisions check" as well. You need 5 decision points for the peace option to work (maximum is 7), together with the red/blue answers.
Three of these decision points are made in ME2 so you're always missing at least 1 point if you haven't played ME/ME2.

I didnt play ME, because I decided to play on my ps3 instead of Xbox. I was 100% paragon in ME2' completed all loyalty missions, and rewrote the heretics. I saved the Admiral and disabled the geth server. I was able to broker peace between the geth and quarians. Just thought I'd let you know you don't have to have destroyed the heretics to win that argument.

FYI, I rewrote the heretics in ME2 but met the other conditions (both Tali and Legion loyal, enough paragon to talk things out with them, Tali not exiled, both ME3 Rannoch side missions completed) and was able to peacefully resolve the Quarian-Geth war. I had both a charm and intimidate option to do it with, and picked the charm option (rally the quarian fleet).